The present invention relates to a precipitated calcium carbonate product.
The invention also relates to a process for preparing such a calcium carbonate product.
In recent years, particularly since 1990, great interest has been focused on precipitated calcium carbonate as a filling and coating material for paper and plastics. A large number of patents has been published relating to this theme because precipitated calcium carbonate (PCC) has proved a many-sided paper filling and coating pigment. PCC provides good opacity and brightness and protects acidic papers against embrittlement and yellowing caused by aging.
The main forms of existence of PCC are calcite, aragonite and vaterite, the first one having a mainly cubic, vaterite an amorphous and spherical, and aragonite an oblong, even acicular, basic crystal form. Intermediate crystal forms are also known and termed rosettes and/or scalenohedral crystals. These mainly resemble a rose bloom in shape.
As a filling or coating agent for paper, finely divided PCC is particularly well suited. The preparation thereof is described, among others, in WO Published Application No. 96/23728 which shows how minute, mainly spherical PCC particles can be produced by means of a causticizing reaction, the dimensions of such particles being 0.2 to 0.4 micrometers. In a solution described in US Pat. No. 4,367,207, fine PCC material is prepared by keeping the temperature of the reaction medium at a value below 18xc2x0 C.
In the prior art, attempts have also been made to obtain finely divided crystals and/or flocs thereof by using auxiliary chemicals. The retention of PCC can be improved by crystal flocculation in connection with paper manufacture. Thus, U.S. Pat. No. 5,332,564 teaches how fine pigment is prepared by adding a small amount of sugar into the water used for slaking the lime. The use of sugar for preventing crystallization has mainly been known in the context of the production of concrete.
One of the main disadvantages of PCC in paper manufacture is it reactivity and disintegration under acidic conditions, such as in the circulation water of a paper machine, because acids which are stronger than carbonic acid are always present. In order to eliminate this problem, the PCC particles have been treated by letting them react to a certain degree with, e.g., phosphoric acid and/or known derivatives thereof. Such inventions have been described, for instance, in U.S. Pat. Nos. 4,219,590 and 4,927,618. In the case of finely divided particles, the further problem is usually present that they are difficult to filter because the particles easily block the pores of filters.
The aim of the present invention is to remove the drawbacks of the prior art and to obtain an entirely novel type of PCC product.
As stated above, by the method described in WO Published Application No. 96/23728, minute PCC particles which are almost equal in size can be obtained. In the present invention, it has surprisingly been found that these can be joined together to form regular polynuclear agglomerates or botryoidal bunches whereby their separation from reaction suspensions is essentially facilitated. The agglomerates according to the invention are spherical and contain several spherical calcium carbonate particles which are attached to one another and have a particle size of about 40 to 400 nm, typically about 40 to 100nm. The particles of precipitated calcium carbonate according to the invention are formed into polynuclear bunches or agglomerates by adjusting the Z potential of the particles to a value from xe2x88x921 to xe2x88x9220 mV. This can be achieved, e.g., in the case of an (alkaline) slurry produced by causticizing by adjusting the pH of the particle slurry to a value from 6.5 to 9.5, preferably from 7.5 to 9.3, in a box filter. Next, the slurry is stirred advantageously in a shock mixer such that the differences in peripheral velocity imposed on the slurry are in the range from 50 to 200 m/s. Correspondingly, a slurry can be formed from the product obtained from carbonization, and its pH is adjusted to a value within a suitable pH range.
In more detail the calcium carbonate products according to the invention are characterized by being in the form of polynuclear spherical agglomerates containing a number of spherical calcium carbonate particles (or aggregates) which are attached to each other and have a particle size of approximately 40 to 200 nm.
The process for its part is characterized by calcium oxide containing starting material being reacted with carbonate ions to form calcium carbonate, the Z potential of the calcium carbonate is adjusted to xe2x88x921 to xe2x88x9220 mV in a suspension, and the calcium carbonate is recovered in the form of agglomerates whose size is approximately 0.1 to 10 xcexcm and which contain calcium carbonate particles (or aggregates) having a size of approximately 40 to 200 nm.
The invention offers considerable benefits. Thus, as already stated above, the separation of calcium carbonate crystals in aggregate or agglomerate (botryoidal bunch) form from a suspension by filtering is easier and less costly than that of conventional fine crystals. In addition, bunches according to the invention are rich in internal reflecting surfaces which improve the opacity of pigments and fillers. In this context it may be mentioned that small foam beads of polystyrene, among others, have previously been suggested for paper coating because they contain a large number of micronic reflecting surfaces within the particle. The use of, among others, calcinated kaolin and structured pigments is based on the same phenomenon. These structured pigments are such pigments onto whose surface the same or a different material has been inoculated in different particle sizes. In this manner it is possible to increase the diffuse reflection of light which increases the likelihood of a ray of light impinging on a reflecting surface. By means of the present invention a calcium carbonate cluster can be obtained with the above-mentioned numerous internal reflecting surfaces which have been brought about essentially without using chemicals. The agglomerates hold together due to van der Waals forces. The product is excellently suited for use in the coating and filling of paper because no chemicals are used, wherefore the paper is flexibly deformed in connection with, e.g. supercalendering.